[Haskell-beginners] Pattern match(es) are overlapped ... but I do not see that they do

[Brent Yorgey]

On Sat, Aug 31, 2013 at 11:39:22PM -0400, Michael Orlitzky wrote:
> On 08/31/2013 11:07 PM, Edward Z. Yang wrote:
> > Well, the trouble is at the source language level there
> > is no way to tell if some_var is /actually/ a constant, or
> > some complicated expression.  So you could do guards:
> > 
> >     case res of
> >       _ | res == wxID_CANCEL -> True
> >         | res == wxID_NO -> False
> >         | ...
> > 
> > The suggestion to use an ADT is, you write a helper fucntion
> > which does this case-split first, and then you do regular pattern
> > matching on the result.  If you need to do this multiple times,
> > it saves you a bunch of typing; it also gives you one place
> > to write the error code when the integer is not one of these
> > three values.
> > 
> 
> In a perfect world, these constants would be defined as part of an
> enumeration type, correct? For example,
> 
>   data WxId = WxIdCancel | WxIdNo | WxIdYes ... deriving (Enum)
> 
> in which case the original attempt would have succeeded since it would
> be matching on a constructor.
> 
> However, unless there are constants defined for 0,1,... this approach
> won't work automatically -- the derived Enum instance starts at zero and
> increments by one. The library would have to define a custom Enum
> instance and it would add a good bit of code.

In this case I wouldn't recommend making a custom Enum instance
(because the Enum class makes all sorts of assumptions that would be
hard to satisfy); I would just make a pair of functions

  wxIdToCode :: WxId -> Int
  wxCodeToId :: Int -> Maybe WxId

-Brent